This invention relates to a process for producing an ambient temperature molten salt to be used as an electrolyte for a secondary battery, etc.
An ambient temperature molten salt comprising a 1,3-dialkyl- or 1,2,3-trialkylimidazolium halide or N-alkylpyridinium halide and an aluminum halide is liquid at ambient temperature, has been known to exhibit considerably high conductivity and expected greatly as a new electrolyte which is different to great extent from organic and inorganic electrolytes of the prior art.
For example, Gifford et al proposed a secondary battery comprising an ambient temperature molten salt formed from a 1,2,3-trialkylimidazolium halide and an aluminum halide as electrolyte (Japanese Provisional Patent Publications No. 3669/1985 and No. 133670/1985). Further, Furukawa et al proposed a secondary battery comprising an ambient temperature molten salt formed from 1-ethyl-3-methylimidazolium chloride and aluminum chloride as electrolyte (Japanese Provisional Patent Publication No. 165879/1987). Also, Kobayashi et al proposed a secondary battery comprising an ambient temperature molten salt formed from 1,3-dialkylimidazolium halide and a metal halide of Group IIIa of the periodic table as electrolyte (Japanese Provisional Patent Publication No. 136180/1985). Moreover, Takahashi et al proposed an electric aluminum plating method using aluminum chloride and N-butylpyridinium chloride as ambient temperature molten salts, and confirmed that the method was cheap and high stability as compared with the conventional electric aluminum plating method (Japanese Provisional Patent Publications No. 70592/1987 and No. 70593/1987).
It has been generally known that an ambient temperature molten salt composed of a 1,3-dialkyl- or 1,2,3-trialkylimidazolium halide or N-alkylpyridinium halide and aluminum chloride, for example, when an alkylimidazolium halide is exemplified, dissoviate into ions as shown by the following schemes. The ion species change with the molar ratio of the both compounds, and ion dissociation occurs as shown by the scheme (1) at a formulated molar ratio of 1:1, and as shown by the scheme (2) at a formulated molar ratio of 2:1. ##STR1## wherein R.sub.1 and R.sub.2 each represent lower alkyl groups, and R.sub.3 represents a hydrogen atom or a lower alkyl group.
In the above example, halogen is chlorine, but similar reactions may be considered to proceed when halogen is bromine or iodine.
In the following, the step of forming an ambient temperature molten salt according to the scheme (1) or the scheme (2) from an aluminum halide and a 1,3-dialkyl- or 1,2,3-trialkylimidazolium halide or N-alkylpyridinium halide is abbreviated as the complex forming step.
In the prior art, the complex forming step is generally the solid mixing method in which an ambient temperature molten salt is produced while mixing gradually an aluminum halide which is a solid and a solid such as 1,3-dialkyl- or 1,2,3-trialkylimidazolium halide or N-alkylpyridinium halide in a glove box under N.sub.2 atmosphere (for example, see Electrochemistry 54, (3), p. 257), as an example using N-butylpyridinium chloride).
The solid mixing method of the prior art in producing an ambient temperature molten salt on an industrial scale from a 1,3-dialkyl- or 1,2,3-trialkylimidazolium halide or N-alkylpyridinium halide and an aluminum halide involves some problems.
As the first point, complex forming reaction is an extremely great exothermic reaction, and it may be pointed out that heat control is extremely difficult in the solid mixing method. For example, Takahashi et al proposed a method of cooling in a dry ice-methanol bath for removal of heat (Electrochemistry, 54, (3), p. 257), but it cannot be an industrial method. This is because it may be considered that, in the solid mixing method, transmission of heat is poor to cause local heat generation, further explosive temperature elevation to occur, whereby the starting materials and (or) the product may be thermally denatured to deteriorate remarkably the characteristics of the molten salt. In fact, when the reaction amount is increased, it was observed that variance in characteristics became greater.
As the second point, for control of heat generation amount in the complex forming reaction, it may be conceivable to add little by little one of the starting materials, but since the characteristics of the molten salt are markedly deteriorated by moisture, the starting materials and the product are required to be handled under absence of moisture, and therefore handling of a small amount of a solid in dry atmosphere will obstruct markedly efficient production.
Also, as the third point, in the solid mixing method, there may be mentioned the drawback that no sufficient stirring operation can be performed to take a long time for the complex forming step.